<i> so if i went with 1.5 - 2 feet of 4 inch pipe and a sprinkler valve would it be able to take 90-100 psi. i dont think i could make a home made piston untill i learn more about how they work. i also dont have a lot of money to spend on a supavalve. if i did decide to make a home made valve would it work any better than a sprinkler valve and would it be able to withstand more psi</i>

A home made 2" piston or floating diaphragm valve, if it works right, would blow the sox off of any 1" sprinkler valve. They have 6 times the airflow.

Joel's 2" Supah is rated 125 psi, I believe. The 2" diaphragm valves I use work well to about 110 psi. Sprinkler valves are good to over 100 psi.. I think.

first off, is your 4" pipe pressure rated? if it isn't, don't use it for this, or you could be seriously injured. if it is pressure rated, it will say so right on the side of the pipe. i believe that 4" sch 40 pipe is rated to 220 psi. pneumatic guns may contain twice as much pressure as combustion guns, so you really want to do this right. when you start doing the math, the forces that are in play here are pretty frightening. if 4" pressure rated pipe is hard to find, 4" pressure rated fittings are at least twice as rare. fittings tend to be stronger than pipe (in my experience) and i have used sch 40 cellular PVC fittings without problems, though i wouldn't recommend it. you should do the safest job possible on this, as it is your health and well being on the line.

ok, safety talk done. 4" pipe holds 4x as much air as 2" pipe for the same length, so if you used 2 ft of 4" chamber and 4 ft of 2" barrel, you would still have a 2:1 ratio, which would work nicely. larger C:B ratios are better, though as the ratio increases, the gain decreases. (what i am about to say assumes no restrictions from a valve, it is just for an understanding of C:B ratios. in reality, valves screw up these numbers significantly. the idea still holds though.) for instance, with a 2" barrelled gun with a 2:1 ratio charged to 100 psi, there will be 314 pounds of force on your projectile when the valve opens. when the projectile is just leaving the barrel, the force behind it has dropped to 209 pounds. with a 4:1 C:B ratio, there is also a 314 pound force behind the potato when the valve opens, but when it leaves the barrel, there is a 250 pound force pushing it.

the best solution to the question you asked is to make an over-under gun rather than the linear combustion gun you probably have experience with. decide the maximum length you would like your gun to be, and use that length for your barrel. make your chamber as long as comfort permits (using 2" and 4" pipe, it should be at least 1/2 as long as the barrel), and suspend it below the barrel with hose clamps and wood or plastic braces. (2 hose clamps can be hooked together to go all the way around.) connect the two with plumbing and poof, you've got a gun.

I have a compressed air cannon as well. It has 4 barrels; 3 x 48, 72 x 2, 84 x 1 3/4, and 84 x 1 1/2. I have found the best cannon blast affects from the larger barrels. The 3" barrel sounds awesome and about 4 oz of water and any projectile presents a great visual too. Velocity is high, but not that fast - perhaps 400 fps with 1274 cuin tank charged to 90 psi. The longer barrels report excellently with a crisp, almost mettalic sound! I fired a 1" super ball out of the 1 3/4 inch barrel at 60 psi - I heard the report, but didn't see the ball. I heard it a bit later smacking around in the woods though. It was really moving... calculated 735 mph.

I guess I should have read better. You wanted to know chamber size - not barrel... sorry. I am using a combined chamber size of 96 inches. My cannon has two chambers that are side-by-side and empty into a single union flowing towards the barrels. Total volume is around 1274 cu in (chambers, pipes, tubes, et cetera). I have found that this amount of chamber volume gives you a lot of room to play with and is somewhat over kill. For example: I get great blasts, and speed with my set up. Doubling the volume would result in little improvement with my existing barrels. However, if I make longer barrels, estimate velocity with the larger chamber size will increase substantially. I suggest you get a caculating program to help you out. I have one that I found through one of spudtech.com's links called "sgtcmodel2000.XLS." It is simple to use.

<blockquote id="quote"><font size="1" face="tahoma,verdana,arial" id="quote">quote:<hr height="1" noshade id="quote">Originally posted by pcitizen[br]I have a compressed air cannon as well. It has 4 barrels; 3 x 48, 72 x 2, 84 x 1 3/4, and 84 x 1 1/2. I have found the best cannon blast affects from the larger barrels. The 3" barrel sounds awesome and about 4 oz of water and any projectile presents a great visual too. Velocity is high, but not that fast - perhaps 400 fps with 1274 cuin tank charged to 90 psi. The longer barrels report excellently with a crisp, almost mettalic sound! I fired a 1" super ball out of the 1 3/4 inch barrel at 60 psi - I heard the report, but didn't see the ball. I heard it a bit later smacking around in the woods though. It was really moving... calculated 735 mph.

I have a hard time believing that. Using GGDT with your given specs and assuming you have a valve that has a Cv of 100, opens in 1ms, and remains open for 1000ms. (That Cv and open time are rediculously optimistic.) I can only attain a MAXIMUM muzzle velocity of 648 fps with a 10 gram projectile (probably on the low side.)
441 mph is a far cry from your said 735 mph (1078 fps.) If I am somehow mistaken or have made a grave error, please enlighten me.